Controllable compressor cooling installation

ABSTRACT

A novel controllable compressor cooling installation is disclosed, the installation being provided with a primary coolant circulation path from a compressor, through a condenser, a shutoff valve, in expansion valve, an evaporator, and then back to the compressor. An auxiliary or secondary coolant circulation path is provided from the outlet of the compressor to the inlet of the evaporator, this secondary or auxiliary coolant path containing a second shutoff valve. In the preferred inventive embodiment, a check or relief valve is provided in the primary coolant circulation path between the branchoff of the auxiliary path and the condenser. Importantly, both shutoff valves in the primary and in the auxiliary coolant paths can be controlled in a time-delayed fashion with respect to one another by means of a two-stage control process. The control is such that, in each instance, the shutoff valve in the primary coolant circulation path closes first and, thereafter, the shutoff valve in the auxiliary or secondary coolant circulation path opens.

United States Patent [72] Inventor Ludwig Melion Buchrain, Oberrohrdorf,Switzerland [21 Appl. No. 860,309

[22] Filed Sept. 23, 1969 [45] Patented Aug. 17, 1971 [73] Assignee LuwaAG Zurich, Switzerland [32] Priority Sept. 26, 1968 [3 3] Switzerland[54] CONTROLLABLE COMPRESSOR COOLING INSTALLATION 5 Claims, 1 DrawingFig.

[52] US. Cl 62/158, 62/196, 62/278 [51] Int. Cl F25b 29/00 [50] Field ofSearch 62/196,

[56] References Cited UNITED STATES PATENTS 2,344,215 3/1944 Soling62/196 3,332,251 7/1967 Watkins Primary Examiner-Meyer PerlinAt!0rneyWerner W. Kleeman ABSTRACT: A novel controllable compressorcooling installation is disclosed, the installation being provided witha primary coolant circulation path from a compressor, through acondenser, a shutoff valve, in expansion valve, an evaporator, and thenback to the compressor. An auxiliary or secondary coolant circulationpath is provided from the outlet of the compressor to the inlet of theevaporator, this secondary or auxiliary coolant path containing a secondshutoff valve. In the preferred inventive embodiment, a check orreliefvalve is provided in the primary coolant circulation path betweenthe branchoff of the auxiliary path and the condenser. Importantly, bothshutoff valves in the primary and in the auxiliary coolant paths can becontrolled in a time-delayed fashion with respect to one another bymeans of a two-stage control process. The control is such that, in each]instance, the shutoff valve in the primary coolant circulation pathcloses first and, thereafter, the shutoff valve in the auxiliary orsecondary coolant circulation path opens.

PATENTED mm mm 35:99AM) LUDW/G MEL lO/l/ INVEN 1 OR ATTORNEYSCONTROLLABLE COMPRESSOR COOLING INSTALLATION BACKGROUND OF THE INVENTIONThis invention generally relates to cooling installations andparticularly concerns a controllable compressor cooling installation ofthe type which is provided with a primary coolant circulation path froma compressor, through a condenser, a shutoff valve, an expansion valve,an evaporator, and then back to the compressor as well as a secondary orauxiliary coolant circulation path extending from the outlet of thecompressor to the evaporator inlet.

In some cooling installations, it is necessary to effect operationalcontrol over the cooling output thereof. A typical example of aninstallation wherein such control is necessary, is the so-calledclimatic control installation wherein the cooling requirements thereofare highly variable in dependence upon both the month and the time ofday and wherein the full capacity of such installation is demanded onlyduring relatively short portions of the total operational period.Accordingly, in this instance, as well as in many other applications,economic and operational considerations dictate only a partial output orload operation of the installation.

In this respect, a number of control possibilities are known to the artwherein the cooling output of a compressor cooling installation iscontrolled or alternatively temporarily reduced. However, the prior artsolutions, as will be discussed hereinbelow, are either uneconomical, orare unsatisfactory in other respects.

Perhaps the simplest technique wherein control over the cooling outputof a compressor cooling installation can be effected is to switch thecompressor motor on and off as required. However, when so doing,substantial variations in temperature can occur, and even moreimportantly, an undesirable peak demand is placed upon the electricalfeed system upon each successive startup of the electric drive motor. Ifa high switching frequency is desired such as for obtaining aquasi-steady cooling control, the danger of overheating the electricmotor exists due to the repetitive high startup current flows, thisdanger being particularly great when one is utilizing single-phase drivemotors. Larger cooling installations can be provided with a plurality ofparallely working though separately driven compressors which areindividually switched on and off so as to effect control over thecumulative cooling output. In this instance, as was the case in theexample discussed above, each and every switching operation creates apeak load in the system. Furthermore, the application range as well asthe gradation of the cooling control are fixed by the number ofcompressors provided, which number normally is relatively low.' Asanother alternative, control over the rotational speeds of thecompressor drive motor may be effected to achieve gradation of thecooling output. Yet, in this instance, the low end range of control isseverely limited in practice due to the speed characteristics of theelectric motor as well as to the characteristics and properties of thecontrol or governor therefor.

A further known means to effect control over the cooling output ofcompressor cooling installations involves constructional modification ofthe reciprocating compressor so as to reduce the circulation of thecoolant. Such techniques comprise lifting of a control valve, opening abypass valve at individual cylinders of the compressor or changing thedead space or volume therein. By such techniques, the power inputrequirements of the drive motor for the compressor admittedly arecorrespondingly reduced. Yet, when compared and contrasted withoperation of the compressor at full load, the proportional or relativemotor losses actually increase, and the friction losses as well as thecompression losses of the unloaded portions of the compressor remainpractically the same. So as to remove the relatively increased heatlosses generated at the compressor after the condenser, only the reducedflow of coolant is available. Accordingly, the danger of overheating onthe pressure side of the compressor is prevalent. Yet, the coolant whichis mixed with lubricating oil cannot exceed a certain maximumtemperature at any point of its circulation as otherwise the chemicalstability of the coolant mixture is endangered. Thus, a sufficientquantity of coolant must continuously be kept in circulation. In otherwords, the cooling output must not be allowed to decrease below a moreor less significant fraction of the full cooling output.

For the purpose of partial cooling load operation, it is also known inthe art to provide a regulating valve or the like in the suction conduitof the compressor so as to reduce the quantity of coolant which issuctioned in from the evaporator. For similar reasons, as discussed inthe above prior art examples, the danger of overheating the compressedcoolant-oil-mixture is highly prevalent.

In other known arrangements and with other known techniques for reducingthe cooling output, a gas bypass conduit is provided between the highpressure side and the low pressure side of the coolant circulation path.A regulating valve such as a so-called output regulator or governoroperated by a governor or other control is disposed in the gas bypassconduit. In dependence upon the amount of opening of the regulatingvalve, a portion of the coolant flow is diverted through the bypassconduit whereas the remaining portion of the coolant flow circulates inthe normal or primary circulation path. However, such arrangementsoperate very uneconomically at reduced output, since even when operatingat such reduced output, the entire quantity of coolant is alwaysconducted through the compressor or is otherwise compressed.Furthermore, if the high pressure side of the compressor is directlyconnected with its low pressure side, a great danger of overheatingexists since a portion of the compressed heating gas is not allowed tocool off in the condenser but again directly reaches the suction side ofthe compressor.

SUMMARY OF THE INVENTION Thus, a need exists in the art for a.controllable compressor cooling installation wherein the cooling outputthereof can be controlled in a fashion eliminating the drawbacks of theabove-described prior art. It is the primary object of the in stantinvention to provide such a cooling installation. Additionally thoughequally important objects of the instant invention, are:

The provision of a controllable compressor cooling installation whichoperates economically even when under only a partial load;

The provision of a controllable compressor cooling installation by whichcontrol over the cooling load can be effected to a great extent andthroughout a relatively large range, even down to zero output withoutthe danger of overheating the coolant; and,

The provision of a controllable compressor cooling installation whichdoes not create deleterious electrical load peaks in the electrical feedsystem of the compressor motor.

These objects as well as other objects which will become apparent as thedescription proceeds, are implemented by the novel compressor coolinginstallation which, in accordance with the invention, is characterizedby the provision, in a compressor cooling installation as discussed atthe outset of the specification, of a relief or check valve in theprimary circulation path between the condenser and the branchoff of theauxiliary or secondary circulation path, and the provision of a secondshutoff valve in the auxiliary path. Each of the shutoff valves, in theprimary and in the secondary or auxiliary circulation paths, can becontrolled in a time-delayed fashion with respect to one another bymeans of a two-stage control process in such a manner that, in everyinstance, the shutoff valve in the primary circulation. path closesfirst and,

thereafter, the shutoff valve in the auxiliary or secondary circulationpath opens.

The time-delayed control of the shutoff valves can advantageously beeffected by connecting the shutoff valves via control circuits and thelike to a governor or like regulating device in which a valve controlcommand time delay is preselected. However, according to an alternativeinventive embodiment, a governor or like regulating device operatingwith a time delay can be eliminated and the primary circulation pathshutoff valve can be directly controlled by a governor whereas theshutoff valve in the auxiliary or secondary circulation path can beconnected via a conductor to a pressure sensor responding to reducedcoolant pressure in the evaporator whereby an opening command for thesecond shutoff valve is generated. In this manner, the time-delayedswitching of the shutoff valves as discussed can occur in directdependency upon the pressure in the coolant circulation path so as todetermine the point and switching time without necessitating thepreselection and determination of approximate constant time delays. Ineach, instance, the novel invention in the preferred embodiment thereof,contemplates the utilization of magnetic valves for the shutoff valvesof the system.

Q 7 BRIEF DESCRIPTION OF THE DRAWING I The invention itself will bebetter understood and features and advantages thereof not previouslydiscussed will become apparent from the following detailed descriptionof a preferred inventive embodiment, such description referring to theappended single sheet of drawing wherein the sole FIGURE thereon depictsan exemplary embodiment, in schematic illustration, of the novelinventive controllable compressor cooling installation or system.

DETAILED DESCRIPTION OF A PREFERRED INVENTIVE EMBODIMENT Referring nowto the drawing, the main or primary circulation path of the coolant canbe traced in the manner discussed hereinbelow. A compressor'l driven byan electric motor 2 suctions or draws in coolant via a conduit 18running from an evaporator 8. The gaslike compressed chemical coolantleaves the compressor 1 via a hot gas conduit 10 from where such gaspasses through a relief or check valve 24 into a conduit 12 andsubsequently reaches a condenser'4 whereat the gas is cooled andliquified by cooling water circulation 5. As illustrated, a collector 6for the liquified coolant can be provided after the condenser 4 or, ifdesired, the condenser itself can be constructed so as to include acollector. Thereafter, the liquified coolant passes in known mannerthrough an aftercooler (not illustrated) at the condenser 4 or thecollector 6, respectively.

The liquid coolant then passes through conduit 14 in which acontrollable shutoff valve such as a magnetic valve, and an expansionvalve 7 are successively provided. Thereafter, the coolant flows throughconduit 16 into an evaporator 8 whereat the coolant absorbs heat fromthe environment by evaporation and thus, the surrounding environment iscooled.

The coolant, which is now in vapor or gas form, then passes intocompressor 1 and the continuous cycle repeats.

As will be noted, an auxiliary or secondary conduit 19 branches off fromthe hot gas conduit 10. Conduit 19 is provided with a second shutoffvalve 22 which may likewise be constructed as a magnetic valve, theauxiliary or secondary conduit leading to conduit 16 which feeds theinput to evaporator 8. The operation of the shutoff valves 20 and 22will be discussed hereinbelow.

When it is desired to operate the cooling installation at full coolingoutput, shutoff valve 20 is opened and shutoff valve 22 is closed. Now,the entire quantity of coolant circulates in known manner in theabove-described primary circulation path. Compressor 1 serves tocompress the entire flow of coolant and raise the pressure thereof tothat required to maintain the circulation, the primary pressurereductionof the coolant occurring at the expansion valve 7.

Control over the cooling output in the illustrated inventive embodimentis achieved by means of a two-stage control process wherein the systemis repeatedly and reversibly switched between two different operationalconditions which,

for convenience, will be referred to hereinbelow as work interval andrest interval" respectively. The change or control in the cooling outputis achieved by varying the time relationship between the work intervaland the rest interval, and, as such, it is possible to achieve aquasi-steady control over the cooling output with due considerationbeing given of a switching frequency sufficient for the thermal timeconstant of the embodiment. The range of cooling control will be seen tobe disposed between the extreme and opposite conditions of permanentwork condition" representing full output and permanent rest conditionrepresenting zero output.

Control over the cooling output is exclusively achieved by means of anopen-close control of both of the shutoff valves 20 and 22 while thecompressor 1 continuously operates. The work condition or work interval"corresponds to the abovedescribed normal operation in which shutoffvalve 20 is open and shutoff valve 22 is closed. The transition from thework interval" to the rest interval takes place in every instance in atwo-stage process and is initiated by closing valve 20. Subsequently,while valve 22 is still in its previously closed condition, compressor 1suctions the coolant vapor from the evaporator 8 and feeds such vaporvia conduit 10 and relief'or check valve 24 to the condenser 4 wheresuch coolant is liquified and stored in collector 6. Since an afterflowof coolant to evaporator 8 is impossible with valve 20 being closed, thecoolant pressure in the evaporator 8 quickly drops during thisintermediate control phase. Thereafter, shutoff valve 22 in theauxiliary or secondary conduit 19 is opened and this constitutes thesecond state, thereby achieving a rest condition or rest interval."Accordingly, during the rest interval," a circulation path of low flowresistance via the auxiliary or secondary conduit 19, shutoff valve 22,and evaporator 8, exists between the suction conduit 18 and the pressureconduit 10 of the compressor 1. Accordingly, a relatively low-residualquantity of the coolant circulates in the above-mentioned lowpressurecirculation path while the major portion of the coolant on the condenserside is maintained between the relief or check valve 24 and the shut-offvalve 20. Through suitable dimensioning of the auxiliary conduit 19 andof the valve 22, the coolant flow resistances in the low pressurecirculation path are maintained low such that compressor 1 is onlyslightly loaded with the circulation quantity of coolant during the restinterval, such circulation quantity furthermore being strongly reduced.The power draw of the drive motor 2 is thus correspondingly low.

The rest interval is terminated and the work interval initiated byopening valve 20 and closing valve 22, In the work interval," the entirequantity of coolant is again circulated via the condenser 4, and theevaporator 8, with the compressor 1 being fully loaded and theinstallation operating at its full cooling capacity.

The residual quantity of the coolant which circulates during the restinterval through the auxiliary or secondary conduit 19, open valve22,and evaporator 8, functions to conduct away the slight heat lossoccurring in the strongly unloaded compressor 1 as the .drive motor 2therefor continues to run. This heat loss is discharged to thesurrounding environment upon passage of the coolant through theevaporator 8. Throughout this circulation path, the residual quantity ofcoolant permanently remains in a gaslike condition contrary to thenormal or primary circulation through condenser 4 with a full outputoperation. Accordingly, the coolant enters evaporator 8 already in theform of a gas and is not evaporated therein. Thus, during the restinterval" a zero cooling output of the installation is easily obtained.In fact, the cooling output is even' somewhat negative due to theabove-mentioned discharge into the environment of the residual heatloss.

The pressure differential which must be produced by com- 1 pressor 1during the rest interval is determined only by the flow resistancesprevalent in the auxiliary or secondary circulation path through valve22 and evaporator 8. So as to keep these resistances and thus thecompression work during the rest interval" to a minimum, the auxiliaryor secondary conduit 19 as well as the valve 22 are suitably constructedso as to exhibit large passage cross sections to the extent possible.The residual quantity of coolant which remains in circulation during therest interval" is dependent and interrelated with its above-describedfunction of removing the slight heat loss from compressor 1. In anygiven cooling installation, the vapor pressure in the auxiliary orsuction-in conduit 19 or in evaporator 8, respectively, is a partialmeasure of this quantity. In a given installation, it is exemplarystated that the pressure in the evaporator during the rest interval" issuitably reduced to about 1 ata. as compared to 5 ata. during the workinterval" or full cooling output operation. This corresponds to a flowof coolant through the compressor which has been reduced to about 20percent of its full operational value. The power input to the motor 2 islikewise accordingly reduced by about 80 percent during the restinterval.

From the above considerations, it will be apparent that the novelcooling installation can be economically operated at only a lowpercentage of its full cooling output capacity even through a longperiod of time. This is the case since the time average value of theconsumed drive power is reduced approximately proportionally to the timeaverage value of the cooling output or load. If the above-mentionedrelatively low pressure in the evaporator 8 and in the auxiliary branchstream of the coolant, respectively, is correctly effected, then nodanger of coolant overheating results even if the cooling output or loadof the installation is completely reduced to zero. 0n the contrary, thegas and motor temperatures drop even below the corresponding valuesassociated with full output operations.

Structurally speaking, it has been found advantageous to keep the volumeof the heated gas conduit 10 between the compressor outlet and therelief valve 24 to as small an extent as possible. Likewise, the volumeof the auxiliary conduit 19 to valve 22 should be kept at a minimum.This is desirable since, during the transition from work interval torest interval," when the valve 22 is opened after evaporator 8 has beensuctioned off, the coolant gas compressed in the conduit volumediscussed above, relaxes or expands itself via valve 22 and againsomewhat increases the low pressure produced in the evaporator 8.

As schematically illustrated in the drawing, control over the coolingoutput of the installation and of the activation of the shutoff valvesand 22, which valves preferably are constructed as magnetic valves, isachieved by means of a twostage governor or control apparatus 30 viacontrol conductors 35 and 36. The nominal or desired value of thecooling output or for the temperature to be maintained by theinstallation, respectively, is inputted via conductor 32 to the governoror control apparatus 30 whereas the actual value is inputted viaconductor 34, such actual value being sensed by a thermal sensingapparatus 33 which determines the cooling temperature as actuallyobtained in the surrounding environment of evaporator 8. As should beappreciated from the foregoing comments, it is essential that in eachinstance during the transition from work interval" to rest interval, theshutoff valve 20 is first closed and that thereafter, shutoff valve 22is only opened after a certain time delay when the required low pressurein the evaporator 8 has been reached. This time delay can be assumed toat least be approximately constant in every given installation andtherefore, such time-delay can be predetermined and preselected in thegovernor or control device 30. Alternatively, instead of controlling theoperation of shutoff valve 22 in the auxiliary or secondary conduit 19by means of the governor or control apparatus 30, such operation canalso be controlled as illustrated by the conductor 38 by means of apressure sensor 37 disposed at the evaporator. This pressure sensorresponds when the evaporator 8 is progressively suctioned empty afterthe closing of valve 20 to thereby reach the desired value of the lowpressure which has been preselected or built into sensor 37.

The load changes on the compressor 1 which attend the two-stage controlprocess discussed above become correspondingly noticeable in the inputefficiency to the drive motor 2. However, since these load changes occurwhile the machine group continuously runs, the effect upon theelectrical feed system and particularly, the increased current drawduring repeated loads, remains quite tolerable. in any event, such arefar less than would be the case if the machine group were to start eachtime from a stopped. or rest position as is the case in the conventionalknown on-ofl' cooling controls discussed at the outset.

With relatively little additional expenditure and by utilizing arelatively simple governing device or control, control of the coolingoutput in the above-described novel compressor cooling installation canbe effected over the entire range between the full nominal ortheoretical output value and a zero value, all without subjecting themachine or the coolant to deleterious overheating. The particular novelcontrol mode also per mits partial cooling outputs during any desiredperiod of time, all with a high efficiency. Furthermore, the novelcontrol process as described effects no undesirable or impermissiblepeak loads upon the electrical feed system, and, as should be apparent,the novel arrangement and control technique is equally applicable withinstallations having either a reciprocating compressor or aturbocompressor.

As should now be apparent, the objects initially set forth at the outsetof the specification have been successfully achieved. ACCORDINGLY,

What I claim is:

l. A controllable compressor cooling installation, said installationproviding a primary coolant circulation path from a compressor, througha condenser, a first shutoff valve, an ex pansion valve, an evaporator,and then back to said compressor; said installation providing asecondary coolant circulation path comprising an auxiliary conduitdisposed from the outlet of said compressor to the inlet of saidevaporator; check valve means disposed in said primary circulation pathbetween said condenser and the connection branchoff of said auxiliarycon duit; a second shutoff valve disposed in said auxiliary conduit; andtwo-stage time-delay control means for both said first and secondshutoff valves, said control means closing said first shutoff valvefirst and thereafter, opening said second shutoff valve in saidauxiliary conduit.

2. A cooling installation as defined in claim ll, wherein said first andsecond shutoff valves are connected to said control means via controlconductors, and wherein said control means comprises a governing devicehaving a preselected time delay between the generation of a controlcommand for said first shutoff valve and a control command for saidsecond shutoff valve.

3. A cooling installation as defined in claim 1, wherein said first andsecond shutoff valves are magnetic switch valves.

4. A controllable compressor cooling installation, said installationproviding a primary coolant circulation path from a compressor, througha condenser, a first shutoff valve, an expansion valve, an evaporator,and then back to said compressor; said installation providing asecondary coolant circulation path comprising an auxiliary conduitdisposed from the outlet of said compressor to the inlet of saidevaporator; check valve means disposed in said primary circulation pathbetween said condenser and the connection branchoff of said auxiliaryconduit; a second shutoff valve disposed in said auxiliary conduit; andtwo-stage time-delay control means for both said first and secondshutoff valves, said control means closing said first shutoff valvefirst and thereafter, opening said second shutoff valve in saidauxiliary conduit, said control means for said first and second shutoffvalves comprising a governing device for said first shutoff valve insaid primary circulation path, and a pressure sensor means and controlconductor therefor for said second shutoff valve, said pressure sensormeans responding to a reduced coolant pressure in said evaporator so asto generate an opening command for said second shutoff valve.

5. A controllable compressor cooling installation, said in stallationproviding a primary coolant circulation path from a compressor, througha condenser, a first shutoff valve, and expansion valve, an evaporatorand then back to said compresshutoff valve first and thereafter, openingsaid second shutoff valve in said auxiliary conduit, said auxiliaryconduitdefining a low pressure circulation path for throughflow of arelatively low residual quantity of the coolant through said secondarycoolant circulation path while the major portion of the coolant ismaintained between said check valve means and said first shutoff valveof said primary circulation path.

1. A controllable compressor cooling installation, said installationproviding a primary coolant circulation path from a compressor, througha condenser, a first shutoff valve, an expansion valve, an evaporator,and then back to said compressor; said installation providing asecondary coolant circulation path comprising an auxiliary conduitdisposed from the outlet of said compressor to the inlet of saidevaporator; check valve means disposed in said primary circulation pathbetween said condenser and the connection branchoff of said auxiliaryconduit; a second shutoff valve disposed in said auxiliary conduit; andtwo-stage time-delay control means for both said first and secondshutoff valves, said control means closing said first shutoff valvefirst and thereafter, opening said second shutoff valve in saidauxiliary conduit.
 2. A cooling installation as defined in claim 1,wherein said first and second shutoff valves are connected to saidcontrol means via control conductors, and wherein said control meanscomprises a governing device having a preselected time delay between thegeneration of a control command for said first shutoff valve and acontrol command for said second shutoff valve.
 3. A cooling installationas defined in claim 1, wherein said first and second shutoff valves aremagnetic switch valves.
 4. A controllable compressor coolinginstallation, said installation providing a primary coolant circulationpath from a compressor, through a condenser, a first shutoff valve, anexpansion valve, an evaporator, and then back to said compressor; saidinstallation providing a secondary coolant circulation path comprisingan auxiliary conduit disposed from the outlet of said compressor to theinlet of said evaporator; check valve means disposed in said primarycirculation path between said condenser and the connection branchoff ofsaid auxiliary conduit; a second shutoff valve disposed in saidauxiliary conduit; and two-stage time-delay control means for both saidfirst and second shutoff valves, said control means closing said firstshutoff valve first and thereafter, opening said second shutoff valve insaid auxiliary conduit, said control means for said first and secondshutoff valves comprising a governing device for said first shutoffvalve in said primary circulation path, and a pressure sensor means andcontrol conductor therefor for said second shutoff valve, said pressuresensor means responding to a reduced coolant pressure in said evaporatorso as to generate an opening command for said second shutoff valve.
 5. Acontrollable compressor cooling installation, said installationproviding a primary coolant circulation path from a compressor, througha condenser, a first shutoff valve, and expansion valve, an evaporatorand then back to said compressor; said installation providing asecondary coolant circulation path comprising an auxiliary conduitdisposed from the outlet of said compressor to the inlet of saidevaporator; check valve means disposed in said primary circulation pathbetween said condenser and the connection branchoff of said auxiliaryconduit; a second shutoff valve disposed in said auxiliary conduit; andtwo-stage time-delay control means for both said first and secondshutoff valves, said control means closing said first shutoff valvefirst and thereafter, opening said second shutoff valve in saidauxiliary conduit, said auxiliary conduit defining a low pressurecirculation path for throughflow of a relatively low residual quantityof the coolant through said secondary coolant circulation path while themajor portion of the coolant is maintained between said check valvemeans and said first shutoff valve of said primary circulation path.